Sources of Error in Weighing Instruments

 

Environmental Factors – A scale’s accuracy and precision are highly dependent on the environment in which it is installed. Several environmental factors can affect the scales measurement including:

  • Air Currents / drafts – These account for most large random errors. Be sure to use your weighing device in an area free of any drafts or air currents that may affect the weight readout. On high precision analytical balances (0.1mg or better), glass draft shields are required. Care should also be taken when weighing objects that are hot or cold inside a draft chamber. The effect of convection currents can make cold objects appear heavier, and hot objects appear lighter.

  • Air Buoyancy – The upward force, caused by atmospheric pressure. The net upward buoyancy force is equal to the magnitude of the weight of air displaced by an object. Air buoyancy is mostly a concern when weighing objects of relatively low density.

  • Temperature – Spring scales and load cell scales deflect at a lower rate and consequently perform poorly under cold conditions. Most springs and load cells are temperature compensated to counteract this source of error to a degree. The scale should always be used within the manufacturer’s recommended operating temperature. For most scales this is between 32°F and 104°F. When moving a scale from one climate to another, you should allow the internal components to acclimate their new environment before performing calibration.

Zero Error – Occurs when the weighing curve shifts by a constant amount. For the most part, you can avoid this error by using the re-zero function before performing a weighing.

Sensitivity Error – Quotient of the change in an indication of a measuring system and the corresponding change in a value of a quantity being measured. Sensitivity of a measuring system can depend on the value of a quantity being measured increasing linearly with heavier loads. Sensitivity errors can occur from temperature drift, aging, adjusting with an incorrect calibration weight, or incorrect compensation of an off-center load error.

Linearity – This is the ability of a scale’s characteristic curve to approximate a straight line. Linearity can be tested by weighing several test weights of increasing value up to maximum capacity and plotting them as points in a graph. The linearity would be the maximum amount that the points deviate from a straight line going from zero to max capacity.

Random Error – The sample standard deviation of the error (indicated values) for a number of consecutive automatic weighings of a load, or loads, passed over the load receptor, shall be expressed mathematically as: